MAP_STACK Stack Register Checking Committed to -current

Implement MAP_STACK option for mmap(). Synchronous faults (pagefault and
syscall) confirm the stack register points at MAP_STACK memory, otherwise
SIGSEGV is delivered. sigaltstack() and pthread_attr_setstack() are modified
to create a MAP_STACK sub-region which satisfies alignment requirements.
Observe that MAP_STACK can only be set/cleared by mmap(), which zeroes the
contents of the region -- there is no mprotect() equivalent operation, so
there is no MAP_STACK-adding gadget.
This opportunistic software-emulation of a stack protection bit makes
stack-pivot operations during ROPchain fragile (kind of like removing a
tool from the toolbox).